The present invention relates generally to scanning electron beam systems for X-ray production in a computed tomography X-ray transmission system, and more particularly to reliably controlling the shape and position of an electron beam-spot as it is scanned across target to produce X-rays in such systems.
A century ago, mathematician J. Radon demonstrated that a two-dimensional slice of a three-dimensional object may be reproduced from the set of all of its projections. Computed tomography (CT) X-ray systems generate a set of X-ray beam projections through an object to be examined. The resultant detected X-ray data are computer processed to reconstruct a tomographic image-slice of the object.
Conventional CT systems subject the object under examination to one or more pencil-like X-ray beams from all possible directions in a plane. The X-ray data may be generated in fan beam format (as is the case for the present invention), or in parallel beam format. In a fan beam system, the X-rays radiate from a source and are collected in a fan. By contrast, in a parallel beam system the X-rays are all parallel within a view. In either system, a view is one projection of the object onto the detectors, and a scan is a collection of all of the views.
In a fan beam scanning electron beam system such as described in U.S. Pat. No. 4,521,900 to Rand, or U.S. Pat. No. 4,352,021 to Boyd, an electron beam is produced by an electron gun and is accelerated downstream along the z-axis of an evacuated chamber. Further downstream a beam optical system deflects the electron beam about 30xc2x0 into a scanning path, with azimuthal range typically about 210xc2x0. The deflected beam is then focused upon a suitable target, typically a large arc of tungsten material, which produces a fan beam of X-rays.
The emitted X-rays penetrate an object (e.g., a patient) that is disposed along the z-axis and lying, typically upon a couch, within a so-called reconstruction circle. X-ray beams passing through the object are attenuated by various amounts, depending upon the nature of the object traversed (e.g., bone, tissue, metal). One or more X-ray detectors, disposed on the far side of the object, receive these beams and provide signals proportional to the strength of the incoming X-rays. A collimation system is typically disposed between the X-ray detectors and the X-ray target (or targets).
Typically the output data from the detectors are processed using a filtered back-projection algorithm. Detector data representing the object scanned from many directions are arranged to produce image profiles for each scan direction. Since the X-rayed object is not homogeneous, these profiles will vary in intensity with the amount of radiation detected by the various detectors on the various scans. The convoluted data from the various projections are then superimposed, or back-projected, to produce a computed tomographic image of the original object. The thus processed data are used to produce a reconstructed image of a slice of the object, which image may be displayed on a video monitor.
Systems similar to what is described in the above patents to Rand or Boyd are manufactured by Imatron, Inc., located in South San Francisco, Calif. These systems are termed xe2x80x9cshort scanxe2x80x9d because the views used for reconstructing an object image cover 180xc2x0 plus the fan beam angle (about 30xc2x0), e.g., about 210xc2x0 total, rather than a full 360xc2x0. In a scanning electron beam CT system, the 210xc2x0 angle implies that the target and detector must overlap, which is to say occupy the same space azimuthally.
The quality of the reconstructed image produced by a CT system is highly influenced by the position (measured in time and in radius) and by the shape of the electron-beam spot as it is scanned along the arc-shaped X-ray emitting target. Prior art techniques that attempt to monitor the position and shape of the preferably ecliptically-shaped electron-beam spot are known. U.S. Pat. No. 4,631,741 to Rand and U.S. Pat. No. 5,224,137 to Plomgren, each of which is incorporated herein by reference, disclosed beam-spot monitoring systems in which devices such as xe2x80x9cWxe2x80x9d-shaped and xe2x80x9cZxe2x80x9d-shaped wires were disposed on a target within the vacuum chamber housing, near but separate from the actual X-ray producing arc-like target. These devices produced beam-spot timing and beam elliptical radius data that could provide some indication as to the quality of the beam-spot along the actual arc-like target.
Unfortunately, because the Rand-Plomgren type devices were on a separate target, the data they provided was available only during a calibration step after the beam scan was completed. Thus, data were not provided from these devices in real-time during the actual scan, and would not be available, for example, with a patient-in-position upon the couch. At best, electron beam-spot shape could only be inferred for the actual X-ray emitting arc-like target from data provided by the phantom-like Rand-Plomgren devices. Further, obtaining meaningful information from these devices required operator intervention and calibration that required halting operation of the scanning system, and was therefore performed typically only on a weekly basis. Understandably, since the xe2x80x9cWxe2x80x9d or xe2x80x9cZxe2x80x9d-shaped wire devices were inside the scanner system vacuum chamber housing, accessing these devices for maintenance and calibration was time consuming.
In summary, for use with electron beam scanner systems there is a need for a method and apparatus to monitor in real-time at least electron beam-spot position and preferably also beam shape during actual acquisition of data with a patient-in-place, preferably with a refresh or update rate exceeding once per ms. Preferably such method and apparatus should utilize much of the configuration already in place for an electron beam CT system and should not require generation of additional X-ray beams. Such apparatus should preferably be readily maintainable and should not interfere with normal operation of the electron beam scanner system.
The present invention provides such a method and apparatus.
In a typical electron beam computed tomography scanner system, the collimation process actually discards the vast majority of generated X-rays. The present invention recognizes that some of the normally discarded X-rays could be used to provide real-time information as to one or more characteristics of the electron beam-spot, such as the position and shape and timing of the electron beam-spot as it is scanned along the source X-ray emitting target, even with a patient-in-place. Advantageously, such X-rays may be collected and used without interfering with normal operation of the CT system.
The CT system is slightly modified to include an extra collimation system. The extra collimation system preferably includes a series of phantom-like objects disposed between the X-ray emitting target and separate detectors disposed to receive at least some of the waste or discarded X-rays not used to acquire an image of the object or patient under X-ray examination. The system of separate detectors is referred to herein as the shielded detector system, e.g., the system is shielded by the collimation system, and it is not necessarily the same detector system that detects X-rays that pass through the object or patient under X-ray examination.
Preferably, construction of the detectors and the phantom-like objects permits gathering information as to beam-spot position (i.e., timing and radius) and preferably also the beam-spot shape, as X-rays are blocked or not blocked from reaching the detectors by the phantom-like objects. As the beam-spot scans along the X-ray producing target, this beam-spot characteristic information may be imparted to multiple detectors substantially simultaneously. This capability can enhance the nature and quality of the information, and provide additional information due to the different apparent angles at which the detectors receive the X-rays. Without limitation, such additional information can include position of the X-ray emitting target. Density of the phantom-like objects is such that there are several objects per detector, which permits obtaining several timing measurements per detector, with a preferably sub-ms overall measurement rate.
The computer system controlling operation of the scanning beam computed tomographic system uses the beam timing, radius, and target position data to make real-time corrections to the electron beam-spot in closed-loop feedback fashion. In this fashion, quality of the computed tomographic image of the patient or object under X-ray examination may be improved, and prior art electron beam-spot sensing devices such as xe2x80x9cWxe2x80x9d-shaped wires may be dispensed with.
Other features and advantages of the invention will appear from the following description in which the preferred embodiments have been set forth in detail in conjunction with the accompanying drawings.